In the field of color photography, couplers, such as cyan, yellow, magenta or the like, are incorporated in a silver halide emulsion using various methods, and coated on a support to prepare a color photographic material, which is imagewise exposed and subjected to a series of photographic processing steps to form images.
The fundamental steps in photographic processing are a color-developing step and a silver-removal step. In the color-developing step, silver halide in an exposed silver halide color photographic material is reduced by color developing agents, thereby forming a silver image and, subsequently, the oxidized color developing agents react with the couplers to provide dye images. Then, the color photographic material is subjected to a second step to oxidize the silver formed in the preceding step with an oxidizing or bleaching agent. The thus oxidized silver is then dissolved with a complexing or fixing agent for silver ion to remove the silver from the photographic material. The photographic material then contains only dye images.
That process is applicable to the developing of photographic color negatives, photographic color positives, photographic color plates, and color cinematographic films. The developers currently and previously used in the trade to process such emulsions contain, as the color developing agent, an N,N-disubstituted p-phenylenediamine. In the course of color development, the color developing agent reacts with phenolic or active methylene couplers that are incorporated in sundry layers of the emulsion on a transparent base or paper and forms non-diffusing dyes that constitute the final image.
Such color developing agents, when in contact with exposed silver halide, distribute in three separate emulsion layers, undergo oxidation to quinone diimines which, in turn, react with the three different color couplers, each of which is present in a different emulsion layer. The amount of quinone diimine formed is directly proportional to the amount of silver halide that has been exposed and, therefore, determines the amount and the intensity of dye that is formed. This process is known as oxidative coupling. The combination of the layers of yellow, cyan, and magenta dyes in amounts determined by the exposed silver halide can be made to reproduce, with a high degree of faithfulness, the colors of the photographed subject.
From a practical point of view, the choice of developing agent is limited because of the characteristics demanded of the dyes, among which are included optimum spectral characteristics and resistance to fading. Also, because the most widely distributed color films and papers are made by and under the control of several manufacturers, the color developing agents must be specifically tailored to the existing color couplers present in such emulsions in order to form the correct dyes. Another required characteristic is non-allergenicity.
One particular N,N-disubstituted p-phenylenediamine color developing agent has gained wide acceptance in the trade and is the color developing agent recommended by the manufacturers for color developing their color papers. This is 4-amino-N-ethyl-N-beta methanesulfonamidoethyl-m-toluidene, hereinafter denominated "AEMT". This compound also is known as 4-amino-3-methyl-N-ethyl,N-beta methanesulfonamidoethyl aniline. AEMT has the following structural formula: ##STR1## As a sesqui-sulfate monohydrate of the free base, the color developing agent is marketed by Eastman Kodak Company under the designation CD-3. As a phosphate monomethanolate of the free base, the color developing agent is manufactured and marketed by Philip A. Hunt Chemical Corporation as a proprietary product under the designation Chemical No. 300.
Another p-phenylenediamine color developing agent has gained wide acceptance in the trade and is the color developing agent recommended by the leading manufacturers for color developing their color negative films. This is 4-amino-3-methyl-N-ethyl-N-beta hydroxyethyl aniline, hereinafter denominated "AMEHEA". The sulfate salt is commonly used in practical color developer formulations.
Other N,N-disubstituted p-phenylenediamine color developing agents have been proposed, but none has found as wide an acceptance as AEMT and AMEHEA. Typical of other N,N-disubstituted p-phenylenediamine color developing agents that form non-diffusing dyes with phenolic and active methylene couplers are N,N-diethyl-p-phenylenediamine; 4-amino-3-methyl-N-diethyl aniline (known in the trade as "ADAT"); and similar N,N-disubstituted p-phenylenediamine color developing agents.
With these developing agents singly used in the respective processes noted, there are specific processing parameters established to produce standard photographic response characteristics accepted by the photographic industry. These parameters, including processing time, temperature, replenishment rate, and capacity, limit the speed and changes in them may increase the cost of processing photographic material. Improvements in one parameter cannot be made without trade-offs in other parameters.
Current trends in photographic processing are towards shorter processing times, lower replenishment rates, higher developer activities, and more concentrated components. Numerous developer activity-increasing additives to allow reduced time, temperature, or replenishment rate have been tried, including the use of black and white developing agents as accelerators for color development and the use of imidazolium salt derivatives. Numerous disadvantages have been experienced with additives for increasing developer activity. Among these disadvantages are an increase in stain, preferential acceleration of the individual dye layers (leading to color imbalances), insolubility or instability of the concentrates from which the developers are prepared, reactivity with other components of the developer, unacceptable color or odor in the developer, increased viscosity, toxicity, adverse environmental effects, prohibitive cost or availability, dye instability or deterioration, and hue changes of dye. Accordingly, there exists in the art a need for a method of increasing developer activity without the aforementioned disadvantages.